Personally I cannot see any scenario where Putin would use a tactical nuke, even if Crimea was seriously under threat. The US and NATO have made it eminently clear their use is a red line and would be responded to in kind. IMO this would most likely be an ultimatum to leave Ukraine and, if this wasn't done, a massive conventional response against Russia's forces in or postured towards Ukraine, effectively sweeping them from the battlefield.
I still reckon Putin is lining up for some kind of "mission accomplished" BS withdrawal from at least some of the occupied territories. The mood music at last weeks meeting in Kazakhstan strongly suggests his "allies" are getting heartily fed up with the whole enterprise and wish him to conclude something quickly.